THERMAL PHYSICS 

1. What is thermometry

The branch of heat that deals with the measurement of temperature with scientific precision is called thermometry and the instrument used tomeasure temperature is called thermometer.

2. What is principle used in the construction of platinum resistance thermometer?

The electrical resistance of a metal is found to increase gradually and fairly uniform with temperature over a wide range and this principle is used in the construction of electrical resistance thermometer.

3. What are the merits of platinum resistance thermometer

It is very compact. It can be used to measure temperatures over a wide range of temperature. (-200^oC to 1200^oC)

It is free from changes of zero point as pure platinum has always the same resistance at the same temperature

4. Define a thermodynamics system

A thermodynamics system refers to a region in space or a quantity of matter bounded by some closed surface. A system may be a gas contained in a cylinder having a movable piston.

5. What are thermodynamics co-ordinates?

The state of thermodynamics system can be represented by specifying its pressure P, volume V, temperature T and entropy S.These variables are known as thermodynamics co-ordinates of the system.

6. State zeroth law of thermodynamics

The zeroth law of thermodynamics states that if two systems are in thermal equilibrium with a third system, then they must be in thermal equilibrium with each other.

7. State first law of thermodynamics

The amount of heat given to a system is equal to the sum of the increase in the internal energy of the system and the external work done.

That is, dQ = dU + dW, where dQ is the quantity of heat given to a system, dU the change in internal energy and dW the external work done.

8. Define a quasistatic process

A process in which the deviation from the thermodynamic equilibrium is infinitesimally small is known as quasistatic process. It is an ideal concept and the condition for such a process cannot be satisfied rigorously in practice.

9. Define an isothermal process

If a system is perfectly conducting to the surroundings and the temperature remains constant throughout the process, it is called an isothermal process. An isothermal change is represented by the equation, PV = constant at constant temperature.

10. Define an adiabatic process

A process in which neither heat is given to the system nor heat is taken from the surroundings is called an adiabatic process. An adiabatic change is represented by equation, PV^γ = constant, where γ is the ratio of specific heat capacities of a gas.

11. Define a reversible process

A reversible process from the thermodynamical point of view is the one which can be retraced in the opposite direction so that the working substance passes through exactly the same condition as it does in the direct process.

12. Define an irreversible process

Any process which is not reversible exactly is an irreversible process.The processes which cannot be retraced in opposite order by reversing the controlling factors are called irreversible processes.

13. What is an indicator diagram?

A graphical method of studying the isothermal processes and adiabatic processes and representing the behaviour of a system is known as indicator diagram. This helps to understand the performance of heat engines.

14. What are the parts of a Carnot engine?

The carnot engine consists of source of heat energy, sink of heat energy, non-conducting stand, cylinder with piston and working substance.

15. State Carnot’s theorem

Statement I: All reversible engines working between the same two temperatures have the same efficiency whatever be the working substance and quantity of heat absorbed or rejected.

Statement II: Of all the heat engines working between the same two temperatures of source and sink the reversible heat engine has the maximum efficiency possible.

16. State second law of thermodynamics

Kelvin statement based on forward Carnot cycle: It is impossible to get continuous supply of work from a body by cooling it to a temperature lower than that of surroundings

Clausius statement based on reverse Carnot cycle: It is impossible to make heat flow from a body at a lower temperature to a body at a higher temperature.

17. Define one Kelvin on the thermodynamic scale

If an engine works between the steam point and ice point of water and if the area of engine’s indicator diagram is divided into 100 equal parts, the area of each part represents a temperature of 1K on the thermodynamic scale.

18. Define absolute zero based on thermodynamic scale.

The absolute zero on the thermodynamic scale is defined as that temperature of the sink at which no heat is rejected to it and the whole of the available energy has been used up in doing useful work and the engine will have 100% efficiency.

19. Define entropy

The entropy of system is defined as dS = dQ/T, where dQ is the amount of heat taken in reversibly by the system at temperature T.

20. State third law of thermodynamics

Planck in 1911 made the hypothesis that not only does the entropy difference vanish as T tends to zero but that the entropy of every solid or liquid substance in internal equilibrium at absolute zero is itself zero.

The third law also implies that it is impossible to reduce the temperature of a system to absolute zero in any finite number of operations. This is called the unattainability statement of third law.

21. What is Joule Thomson effect?

When a gas under constant high pressure passes adiabatically through a porous plug (narrow orifice) it undergoes a large drop of pressure and its temperature changes. This effect is called Joule-Thomson effect or Joule- Kelvin effect.

22. What is temperature of inversion

It is defined as the initial temperature of the gas at which the cooling effect becomes heating effect. At temperature above the temperature of inversion the gases show heating effect. The temperature of inversion is different for different gases.

23. What is the principle of regenerative cooling?

The gas is first compressed and the compressed gas is cooled below its temperature of inversion. The gas is then made to expand through a porous plug and the gas uses its part of energy to overcome the molecular forces of attraction. The gas is thereby cooled and the cooled gas is now made to flow round the incoming gas which on expansion falls further in temperature. This progressive cooling of the gas is continued till it finally liquefies. This is known as the principle of regenerative cooling and has been used for the liquefaction of gases.

24. What is adiabatic demagnetization?

When a paramagnetic substance is magnetized, external work done on it and its temperature rises. But on the other hand when a substance is adiabatically demagnetized, work is done by the substance and its temperature falls. This effect is called magnetic caloric effect. Maximum cooling can be obtained by employing strong magnetic field and low initial temperature.

25. Define thermal conduction

Conduction is the processes of heat transmission from one point to another though the substance without the actual motion of the particle.

26. Define coefficient of thermal conductivity

The coefficient of thermal conductivity is defined as the quantity of heat flowing per second through unit area of cross section of the material when the temperature gradient is unit. The unit of thermal conductivity is Wm^-1K^-1

27. Define thermal diffusivity

The thermal diffusivity is defined as the ratio of the thermal conductivity to the thermal capacity per unit volume.

28. Define thermal radiation

Radiation is the process of heat transmission from one place to other place that does not require the presence of any material medium.

29. Define a black body.

A perfect black body is defined as one which completely absorbs radiation of all wavelengths incident on it and also emits radiation of all possible wavelengths when heated.

30. Define emissive power

The emissive power of a substance is defined as the ratio of the amount of heat radiation emitted by unit area of a surface in one second to the amount of heat radiated by a perfectly black body per unit area of one second under identical conditions.

31. Define absorptive power

The absorptive power of a substance is defined as the ratio of the amount of heat absorbed in a given time by the surface to the amount of heat incident on the surface in the same time.

32. State Kirchoff’s law

At a given temperature the ratio of the emissive power to the absorptive power for all bodies is constant and is equal to the emissive power of a perfectly black body.

33. State Stefan-Boltmann law

The total rate at which a black body emits heat radiation is proportional to the fourth power of its absolute temperature.

34. State Wien’s law

The wavelength corresponding to the maximum energy is inversely proportional to its temperature.

35. State few postulates of kinetic theory of gases

The gas is composed of small individual particles called molecules. The molecules are considered to be rigid, perfectly elastic solid spheres and identical in all respects. They are however of negligible size as compared with their distance apart.

In a gas the distance between the molecules is large as compared to that of a silid or liquid and hence the inter-molecular force of attraction is negligible. Hence in a gas the molecules are in a state of incessant random motion moving in all directions with all possible velocities.